Acousto-optic deflector applications in laser processing of dielectric or other materials

ABSTRACT

A laser processing system for micromachining a workpiece includes a laser source to generate laser pulses for processing a feature in a workpiece, a galvanometer-driven (galvo) subsystem to impart a first relative movement of a laser beam spot position along a processing trajectory with respect to the surface of the workpiece, and an acousto-optic deflector (AOD) subsystem to effectively widen a laser beam spot along a direction perpendicular to the processing trajectory. The AOD subsystem may include a combination of AODs and electro-optic deflectors. The AOD subsystem may vary an intensity profile of laser pulses as a function of deflection position along a dither direction to selectively shape the feature in the dither direction. The shaping may be used to intersect features on the workpiece. The AOD subsystem may also provide rastering, galvo error position correction, power modulation, and/or through-the-lens viewing of and alignment to the workpiece.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional patent application of a Non-Provisionalapplication Ser. No. 13/850,168, filed Mar. 25, 2013. Which iscontinuation of a U.S. patent application Ser. No. 12/790,082, filed May28, 2010. Which is claiming priority of a U.S. Provisional PatentApplication No. 61/181,889, was filed on May 28, 2009, the contents ofwhich are herein incorporated by reference in their entirety for allpurposes.

TECHNICAL FIELD

This disclosure is related to laser processing of dielectric or othermaterials.

BACKGROUND INFORMATION

Laser processing of dielectric and conductive materials is commonly usedto ablate fine features in electronic components. For example, chippackaging substrates may be laser processed in order to route signalsfrom the semiconductor die to a ball-grid array or similar package.Laser processed features may include signal traces, ground traces, andmicrovias (to connect signal traces between package layers). A recentdesign trend incorporates signal and ground traces on a single layer totightly control signal impedance while reducing the number of layers ina chip package. Such an approach may require small feature dimensionsand spacing (e.g., about 10 microns (μm) to about 25 μm), and long tracelengths per package (e.g., about 5 meters (m) to about 10 m). In orderto construct chip packages economically, the speed at which suchfeatures are ablated may be quite high (e.g., from about 1 meter/second(m/s) to about 10 m/s). Certain packages may be processed, for example,in about 0.5 second (s) to about 5 s to meet customer throughput goals.

Another useful characteristic of chip packaging may be to provideintersecting traces with controlled depth variation. For example, groundtraces may branch at several points throughout the pattern. At eachbranching intersection, the traces may be ablated with a desired depthvariation of less than about +/−10%. Normally, if two trenches were tobe ablated at one point, the double exposure of the ablating beam wouldcreate a depth variation of about 100%.

Another useful characteristic of chip packaging may be to providevariable trace widths at different parts of the package to controlimpedance or provide pads for inter-layer connection vias. Trace widthcontrol should be provided with reduced or minimal disruption to thehigh-velocity processing of the main trace.

It may also be useful to process features of arbitrary size and shape,at high speed, with reduced or minimal time used to change the feature'scharacteristics. For example, features may include microvias with avariety of diameters and/or sidewall taper, square or rectangular pads,alignment fiducials, and/or alphanumeric notation. Traditionally, forprocessing features such as microvias, optical systems have beendesigned to provide shaped intensity profiles (e.g., flat-top beams) ofvariable diameter, or purely Gaussian beams. These optical systems mayhave significant time delays (e.g., about 10 milliseconds (ms) to about10 s) when changing laser processing spot characteristics.

Other problems are associated with building a machine to meet theprocessing parameters noted above. For example, traces may changedirection throughout the package due to routing requirements. Whenprocessing traces at high velocity, the variation in trajectory anglemay require high beam position acceleration at very short time scales.Laser processing can easily exceed the dynamic limits of the beampositioner, for example, when running at the high velocities (e.g.,about 1 m/s to about 10 m/s) used for high throughput.

Such accelerations and/or velocities may be difficult to achieve intraditional laser processing machines, which have relied on beampositioning technologies such as linear stages in combination withmirror galvanometer beam deflectors (referred to herein as “galvos” or“galvo mirrors”), along with static (or slowly varying) beamconditioning optics that cannot respond in the time scales used for thistype of processing (e.g., on the order of about 1 microsecond (μs) toabout 100 μs).

The actual ablation process may also be a factor to consider. Laserpulses with high peak power may be used to ablate the dielectricmaterial while minimizing thermal side effects such as melting,cracking, and substrate damage. For example, ultrafast lasers with pulsewidths in a range between about 20 picoseconds (ps) and about 50 ps atrepetition rates of about 5 megaHertz (MHz) to about 100 MHz can processmaterials with high peak power while providing significant pulse overlapto avoid pulse spacing effects. Fiber lasers now commonly provide pulsewidths in the nanosecond region at repetition rates of greater thanabout 500 kiloHertz (kHz). Normally, for a given process condition(ablation depth and width), the “dosage” (power/velocity) applied to theprocessed material should be constant. However, at low velocities, theapplied power may become so low that the peak pulse power may beinsufficient to ablate the material without inducing thermal effects(e.g., melting and charring).

Another processing effect that can reduce ablation efficiency may be theinteraction of the processing beam with the plume of ablated material.Plumes may distort or deflect the beam enough to disrupt the focusedbeam, or cause accuracy issues due to its deflection.

Beam positioner designs may deflect the process beam using galvos. Theintensity profile of the process beam at a workpiece may be Gaussian(for simple focusing of a Gaussian beam), or a shaped intensity profile(e.g., flat-top profile) for beams conditioned by a fixed optic beamshaper.

Systems have been described in which acousto-optic deflectors (AODs)have been combined with galvos to provide high-speed deflection, forexample in U.S. Pat. Nos. 5,837,962 and 7,133,187. However, thesereferences do not describe obtaining the desired performance in advancedbeam positioner designs.

SUMMARY OF THE DISCLOSURE

In one embodiment, a laser processing system for micromachining aworkpiece includes a laser source to generate a series of laser pulsesfor processing a feature in a surface of the workpiece, agalvanometer-driven (galvo) subsystem to impart a first relativemovement of a laser beam spot position along a processing trajectorywith respect to the surface of the workpiece, and an acousto-opticdeflector (AOD) subsystem to effectively widen a laser beam spot along adirection perpendicular to the processing trajectory. The AOD subsystemmay include a combination of AODs and electro-optic deflectors.

In one embodiment, a method is provided for processing features in aworkpiece with a series of laser pulses. The method includes imparting,using a first positioning system, first relative movement of a laserbeam spot position along a processing trajectory. The method alsoincludes imparting, using a second positioning system, second relativemovement of the laser beam spot position along a dither directionrelative to the processing trajectory. The second relative movement issuperimposed on the first relative movement, and the second relativemovement is at a substantially higher velocity than that of the firstrelative movement. The method further includes emitting a firstplurality of laser pulses during the second relative movement toeffectively widen a laser beam spot at the workpiece in the ditherdirection, and varying an intensity profile of the first plurality oflaser pulses as a function of deflection position along the ditherdirection to selectively shape a first feature in the dither direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating operation of an AOD that may beused according to certain embodiments.

FIG. 2 graphically represents AOD diffraction efficiency curves vs. RFpower at various RF frequencies that may be used according to certainembodiments.

FIG. 3 graphically represents an example AOD power linearization curveused to select a desired attenuation according to one embodiment.

FIG. 4 graphically represents AOD diffraction efficiencies vs. RFfrequency that may be used to select a tradeoff between diffractionefficiency and deflection range according to certain embodiments.

FIG. 5 is a block diagram of a system including an AOD subsystem andgalvo subsystem for dithering a laser beam according to one embodiment.

FIG. 5A is a block diagram of a system for beam shaping according to oneembodiment.

FIG. 5B is a block diagram of a system providing slanted process beamsaccording to one embodiment.

FIG. 6 is a flowchart of a method using a least-squares optimizationroutine to determine a set of spot amplitudes over a grid of rasterpoints according to one embodiment.

FIG. 7A graphically represents a desired fluence profile according toone embodiment.

FIG. 7B graphically represents optimized raster amplitudes correspondingto the desired fluence profile of FIG. 7A according to one embodiment.

FIG. 8 graphically represents curves associated with an example AODgalvo error correction filter according to one embodiment.

FIG. 9 is a block diagram of a laser processing system including anauxiliary sensor in a galvo subsystem according to one embodiment.

FIG. 10 is a schematic illustrating example trench patterns processedfor laser direct ablation according to certain embodiments.

FIG. 11 graphically represents curves associated with AOD and galvocoordination according to one embodiment.

FIG. 12 graphically represents curves associated with AOD velocitycompensation according to one embodiment.

FIG. 13 schematically represents parallel processing and region joiningaccording to one embodiment.

FIG. 14 schematically illustrates a tertiary profiling subsystemaccording to one embodiment.

FIGS. 15A, 15B, 15C, 15D, and 15E illustrate signals produced and/orused by the tertiary profiling subsystem shown in FIG. 14 according toone embodiment.

FIGS. 16A, 16B, and 16C illustrate example AOD command sequencesaccording to certain embodiments.

FIGS. 17A and 17B graphically illustrate examples of velocity modulationaccording to certain embodiments.

FIG. 18 graphically illustrates a positioning error with respect to aposition command signal and a resulting AOD position profile accordingto one embodiment.

FIG. 19 is a block diagram of a system for through-the-lens viewingusing an AOD subsystem for raster illumination according to oneembodiment.

FIG. 20 graphically represents AOD diffraction efficiency curvesaccording to an example embodiment.

FIG. 21 graphically represents additional AOD linearization curvesaccording to an example embodiment.

FIG. 22 is a block diagram representing AOD control data flow accordingto one embodiment.

FIG. 23 graphically represents an approach of a butting trench at anintersection according to one embodiment.

FIG. 24 graphically represents a cross-section profile of the buttingand nominal trenches shown in FIG. 23 according to one embodiment.

FIG. 25 graphically represents an optimum intersection with Gaussianbeams according to one embodiment.

FIG. 26 graphically represents a cross-section profile of the optimumintersection with Gaussian beams shown in FIG. 25 according to oneembodiment.

FIG. 27 graphically represents dithered trenches before intersectionaccording to one embodiment.

FIG. 28 graphically represents a cross-section profile of the nominaland butted trenches with dither shown in FIG. 27 according to oneembodiment.

FIG. 29 graphically represents an optimum intersection with ditheredbeams according to one embodiment.

FIG. 30 graphically represents an intersection cross-section withdithered beams (optimum+sensitivity) corresponding to FIG. 29 accordingto one embodiment.

FIG. 31 graphically represents a wide transition edge for improvedposition tolerance (before intersection) according to one embodiment.

FIG. 32 graphically represents a cross-section profile of the nominaland butted trenches with wide transition trenches (before intersection)shown in FIG. 31 according to one embodiment.

FIG. 33 graphically represents an optimum intersection with a widetransition edge according to one embodiment.

FIG. 34 graphically represents an intersection cross-section with widetransition (optimum+sensitivity) corresponding to FIG. 33 according toone embodiment.

FIG. 35 graphically represents a crossed intersection trench with anotch according to one embodiment.

FIG. 36 graphically represents a cross-section profile of the notchedtrench shown in FIG. 35 according to one embodiment.

FIG. 37 graphically represents an optimum crossed intersection accordingto one embodiment.

FIG. 38 graphically represents an intersection cross-section with a widetransition (optimum+sensitivity) corresponding to FIG. 37 according toone embodiment.

FIG. 39 graphically represents a “T” intersection processed with crosstrenches according to one embodiment.

FIG. 40 graphically represents dynamics of dosage and shape control atintersections according to one embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments disclosed herein provide flexible, high-speed beampositioning and conditioning that is economical and feasible. Thisdisclosure describes the use of AODs in combination with linearpositioning stages and/or galvos.

While the example embodiments disclosed herein are directed to AODs,electro-optic deflectors (EODs) may also be used. In certainembodiments, for example, EODs are suitable replacements for some or allAOD pointing (deflection) functions. EODs (when set up for angulardeflection) do not typically modulate power. Thus, in certainembodiments, one or more AODs are used for power modulation and one ormore EODs are used for pointing. Acousto-optic devices that performmodulation may be referred to herein as acousto-optic modulators (AOMs).Other mechanical beam steering technologies such as fast-steeringmirrors (FSMs) may be substituted for the galvo beam positioningsubsystem without loss of functionality.

A laser processing system, according to certain embodiments described indetail below, provides both AOD and galvo positioning. A system thatincludes AOD and galvo beam positioning may provide the ability to tradeoff larger deflection range vs. high diffraction efficiency from the AODby tailoring the power linearization curves for desired operatingcondition.

Certain embodiments provide for dithering the process beam by rapidlyupdating the AOD deflection commands to produce a selected intensityprofile for customized processing of the workpiece. The dithering can beused to change the effective dimensions of a process beam (e.g., itswidth or cross-sections shape), or to create customized spot intensityprofiles for applications such as via formation (top-hat intensityprofiles of arbitrary shape). Dithering may be used, for example, tocreate intersecting ablated features on the workpiece while avoidingundesirable depth variation due to overexposure of portions of theintersection. Intersection processing capabilities disclosed hereinallow for either continuous processing (shaping the beam intensityprofile while processing the main features, without stopping), or customprocessing using a raster approach, which provides the ability to createarbitrarily-shaped intersections that may otherwise be impossible ordifficult to process on-the-fly.

A system with AOD and galvo positioning may also provide for optimizingthe raster pattern (location and intensity of process beam points) toproperly form a desired intersection. Certain embodiments also providefor correcting for galvo positioning error by properly filtering galvoerror signals to match the phase and gain response from the galvo errorto beam position over a selected frequency range, while simultaneouslyfiltering undesirable noise. In an alternative approach to dithering,certain embodiments provide for changing the process beam spot size by“chirping” the AOD acoustic waveform to defocus the beam on apulse-pulse basis.

In addition, or in other embodiments, the operation of galvo beampositioners are coordinated with the operation of AOD positioning toallow the AODs to deflect the process beam for high-bandwidth trajectorycomponents and to allow the galvos to deflect the beam for thelower-bandwidth components, either through separate profiling commandsor through filtering of the main beam trajectory commands High-velocitybeam trajectories may be enabled by allowing the AODs to reduce beamvelocity in a small, local area while not changing galvo velocity, whichenables processing of larger local features at full speed. Similarly,the modulation of process beam power to maintain constant dosage duringfeature processing (independent of beam velocity) allows the galvos torun at full speed in certain sections and to rapidly decelerate to lowerspeeds at other sections to better track the trajectory.

In certain embodiments, multiple workpiece features may be processed inparallel (using AODs to dither between features) to reduce the beampositioner speed and allow higher throughput through parallelprocessing. The intersection processing capabilities provided by AODsmay be used to join portions of the workpiece features processed inparallel to adjoining sections that are not processed in parallel.

AODs may also be used to stabilize beam jitter at little additional costor complexity in the optical train and/or to avoid the undesirableeffects of plume formation during workpiece processing by using the AODsto dither the beam position along the velocity vector of the selectedworkpiece feature. AODs may also be used to simultaneously provide fieldillumination and a reference process beam spot for through-the-lensviewing of and alignment to the workpiece, providing (at little extracost or complexity) the ability to align the process beam to workpiecefeatures with very high accuracy, and also to optimize the focusadjustment for the process beam. AODs may also provide the ability totailor the duty cycle of a process beam such that heat-affected-zoneeffects are minimized.

Reference is now made to the figures in which like reference numeralsrefer to like elements. For clarity, the first digit of a referencenumeral indicates the figure number in which the corresponding elementis first used. In the following description, numerous specific detailsare provided for a thorough understanding of the embodiments disclosedherein. However, those skilled in the art will recognize that theembodiments can be practiced without one or more of the specificdetails, or with other methods, components, or materials. Further, insome cases, well-known structures, materials, or operations are notshown or described in detail in order to avoid obscuring aspects of theinvention. Furthermore, the described features, structures, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Embodiments may include various steps, which may be embodied inmachine-executable instructions to be executed by a general-purpose orspecial-purpose computer (or other electronic device). Alternatively,the steps may be performed by hardware components that include specificlogic for performing the steps or by a combination of hardware,software, and/or firmware.

Embodiments may also be provided as a computer program product includinga non-transitory, machine-readable medium having stored thereoninstructions that may be used to program a computer (or other electronicdevice) to perform the processes described herein. The machine-readablemedium may include, but is not limited to, hard drives, floppydiskettes, optical disks, CD-ROMs, DVD-ROMs, ROMs, RAMs, EPROMs,EEPROMs, magnetic or optical cards, solid-state memory devices, or othertypes of media/computer-readable medium suitable for storing electronicinstructions.

I. AOD Overview

FIG. 1 illustrates the operation of an AOD 100 that may be usedaccording to certain embodiments. The AOD 100 includes a piezoelectrictransducer 110 bonded to a crystal 112. The AOD 100 further includes aradio frequency (RF) drive 113 configured to drive the piezoelectrictransducer 110 to produce an RF frequency acoustic wave 114 (e.g., in afrequency range between about 50 MHz and about 1500 MHz) in the crystal112. An incoming laser beam 115 is diffracted by the acoustic wave 114set up in the crystal 112, with a portion of the input beam power beingdeflected (a “first order” beam 116) and the remainder of the powerbeing undeflected (a “zero order” beam 118). In some embodiments, thefirst order beam 116 is used for processing and the zero order beam issent to a beam dump 122. The first order deflection angle 120 isproportional to the applied RF frequency.

In one embodiment, the incoming beam angle relative to the acoustic wavecolumn is set to the Bragg angle. Setting the incoming beam angle to theBragg angle increases or maximizes the diffraction efficiency, which isthe ratio of first order beam power to input beam power. The relativepower deflected into the first order beam 116 may be approximatelyproportional to the RF power applied by the RF drive 113 at low RF powerlevels. However, the relative power deflected into the first order beam116 may saturate at a high level (e.g., as shown in FIG. 2). In actualoperation, small amounts of power may also be scattered or deflectedinto higher order beams (not shown).

Diffraction efficiencies of AODs can range up to about 95% or more for aproperly designed device with a good (high quality Gaussian) input beam,at a selected RF frequency and amplitude. As the RF frequency varies,the deflected beam angles vary, and the diffraction efficiency fallsbelow its maximum value. AODs can maintain greater than about 90%efficiency over a deflection range equivalent to about 3 to 5 diametersof the focused spot at a workpiece (not shown). Specially designed AODscan achieve even higher diffraction efficiencies through techniques thatsteer the acoustic beam angle as a function of RF frequency.

Two AODs can be combined to create a two-dimensional (2-D) deflectionsubsystem. When placed before the galvos, as discussed below, the twoAODs impart a small beam deflection around the nominal beam positioncreated by the galvos. Such an arrangement is described, for example, inU.S. Pat. No. 5,837,962. When using such an arrangement to process aworkpiece, certain embodiments maintain a constant beam power during AODdeflection despite the variation in diffraction efficiency as a functionof AOD deflection. Maintaining a constant beam power during AODdeflection can be accomplished at high speed (e.g., at update rates usedfor AODs from about 0.1 μs to about 10 μs) by updating (e.g.,modulating) the RF power amplitude as a function of RF frequency. The RFpower modulation has the effect of lowering the diffraction efficiencyin the middle portion of the AOD deflection range such that it matchesor approximates the lowest diffraction efficiency. While matching thelowest diffraction efficiency lowers the efficiency of the deflectionsubsystem, it allows the AODs to be used in applications that usesubstantially constant (or predictable) power over the AOD deflectionrange.

As discussed above, EODs may be used as an alternative to AODs (or inconjunction with AODs) for angular deflection applications. EODdeflectors have capabilities similar to AODs, with limited range (e.g.,equivalent to a few spot diameters at the workpiece), very highbandwidths (e.g., microsecond response times), and high transmissionefficiency. However, the crystals used to implement EOD devices maysuffer from relatively high optical power absorption (e.g., a fewpercent) or significant electrical power dissipation, which may lead toissues with thermal lensing and/or beam pointing drift. Still, for someembodiments (e.g., using low optical power and/or high-transmissionwavelengths), the techniques described below using AODs may beaccomplished with EODs.

II. Power Linearization

To properly use AODs in laser processing applications, according tocertain embodiments, the diffraction efficiency curve is linearized as afunction of RF power and frequency. For predictable operation, anormalized AOD power attenuation command (ranging from 0 to 1) may beused that results in linear attenuation of the first order beam power.FIG. 2 graphically represents AOD diffraction efficiency curves vs. RFpower at various RF frequencies that may be used according to certainembodiments. As shown in FIG. 2, AOD diffraction efficiency curves aregenerally non-linear. Due to the non-linear nature of the diffractionefficiency curves, diffraction efficiency vs. RF power may be mapped,according to certain embodiments, and a linearization function (e.g., apolynomial equation, look-up-table, or similar algorithm) may begenerated that provides the RF power that results in the commandedattenuation.

FIG. 3 graphically represents an example AOD power linearization curveused to select a desired attenuation according to one embodiment. Theexample linearization function illustrated in FIG. 3 may be representedin lookup table form. The linearization function shown in FIG. 3 isvalid for a specified RF frequency, with slight variation for differentRF frequencies. To enable linear operation over a reasonable RFfrequency (and thus deflection) range, certain embodiments use aplurality of linearization tables. The number of linearization tablesdepends on the frequency range applied to the AOD and the precision withwhich the power normalization is maintained. In certain embodiments, thelinearization tables can create linear power regulation within atolerance of about 1%.

In a 2-D AOD configuration, only one AOD may be necessary to control theRF power for power linearization. Modulation of a first AOD's RF powercan provide the control used to linearize the diffraction efficiency ofa second AOD. In certain such embodiments, the second AOD operates nearits saturation point, where changes in the RF frequency have thesmallest effect on diffraction efficiency, which minimizes or reducesthe diffraction efficiency variation as a function of deflection. Ifdesired for certain embodiments, either of the two AODs can be used forpower linearization. In certain embodiments, the first AOD may be usedfor coarse power control and the second AOD may be used for fine powercontrol in order to minimize or reduce the linearization errorsintroduced by quantized RF power commands.

Due to the peaking of the diffraction efficiency curve at an optimum RFfrequency setting, operation at other frequencies may require that theAOD run at a lower optical efficiency. If the AOD is to be operated withconsistent optical output power over a given deflection range, therequested output power may be configured in certain embodiments toremain below the minimum achievable output power over the entiredeflection range. This constraint may be recognized in the design of thelaser processing system, and may guide the selection of the AOD'soperational deflection range. For processes that require very highoptical efficiency, the AOD can be operated within a small deflectionrange (e.g., less than about 5 spot diameters). For processes requiringa larger deflection range (e.g., up to hundreds of spot diameters), themaximum efficiency can be reduced to allow the user to trade offefficiency (e.g., optical power) vs. deflection range. For example, FIG.4 graphically represents AOD diffraction efficiencies vs. RF frequencythat may be used to select a tradeoff between diffraction efficiency anddeflection range according to certain embodiments. FIG. 4 shows thediffraction efficiency response vs. RF frequency shift and the change inminimum diffraction efficiency for an example RF frequency (deflectionamplitude) operating range 412. A high efficiency over a small range isshown at arrow 414 and a lower efficiency over a larger range is shownat arrow 416.

III. Dithering

FIG. 5 is a block diagram of a system 500 including an AOD subsystem 506and galvo subsystem 508 for dithering a laser beam according to oneembodiment. The system 500 includes a laser source 510 for providing aprocess beam 512 to the AOD subsystem 506. In one embodiment, the lasersource 510 includes a pulsed laser source such that the process beam 512comprises a series of laser pulses. In another embodiment, the lasersource 510 includes continuous wave (CW) laser source such that theprocess beam 512 comprises a CW laser beam. In certain such embodiments,the AOD subsystem 506 generates laser pulses from the CW laser beam bydeflecting the process beam 512 at discrete (“pulse”) intervals.

As discussed above, the AOD subsystem 506 deflects a first order beam513 of the process beam 512 at an AOD deflection angle 514 and a zeroorder beam 515 of the process beam 512 to a beam dump 516. The system500 may further include a fixed mirror 518 to deflect the first orderbeam 513 to the galvo subsystem 508, and a scan lens 520 to focus alaser beam spot 522 on or within a workpiece 524. The output of the scanlens 520 may be referred to herein as a focused laser beam 525.

In one embodiment, the AOD subsystem 506 may include a single AOD usedto provide deflection back and forth in a first direction (e.g., adither direction) while the galvo subsystem 508 provides deflection in asecond direction along a processing trajectory 526. For increased speedand versatility, however, the AOD subsystem 506 in the embodimentillustrated in FIG. 5 provides 2-D deflection along an X-axis and aY-axis with respect to a surface of the workpiece 524. In this example,the Y-axis may be referred to as being parallel to the processingtrajectory 526 and the X-axis may be referred to as being perpendicularto the processing trajectory 526. Thus, the X-axis may be referred to asthe dithering direction. The processing trajectory 526 may correspond toa direction, for example, that the system 500 scribes or cuts a trench528 (e.g., under control of the galvo subsystem 508) into a surface ofthe workpiece 524.

To provide the illustrated 2-D deflection, the AOD subsystem 506includes a first AOD 530 to deflect the first order beam 513 in a firstdirection and a second AOD 532 to deflect the first order beam 513 in asecond direction, as the galvo subsystem 508 moves the beam axis along aprocessing trajectory 526. In other words, the movement of beam spotpositions provided by the AOD subsystem 506 is superimposed on themovement of the beam spot positions provided by the galvo subsystem 508.As shown in FIG. 5, the galvo subsystem 508 may also include a firstgalvo mirror 533 and a second galvo mirror 535 to deflect the firstorder beam 513 in both the X-axis and Y-axis directions with respect tothe surface of the workpiece 524.

The orientation of the AOD deflections may not align to the deflectionaxes of the galvo subsystem 508. In general, a coordinate transformationmay be applied to the AOD deflection commands to align the resulting AODdeflections to a desired coordinate frame. This coordinatetransformation may also be a function of velocity, rotating the AODdeflection coordinate frame in order to keep the AOD beam deflectionperpendicular to the processing trajectory defined by the galvosubsystem 508.

With the AOD subsystem 506 included in the system 500, severaloperational modes are enabled. In one embodiment, an operational modeincludes an ability to dither the process beam 512 to effectively widenthe laser beam spot 522 at the workpiece 524. In other words, ditheringthe process beam 512 includes spatially locating a series of focusedlaser beam spots 534 to create geometric features having dimensionsgreater than those of an individual laser beam spot 522 focused by thescan lens 520. For illustrative purposes, FIG. 5 shows the ditheredlaser beam spots 534 as viewed from above the surface of the workpiece524 as the trench 528 is processed in the direction of the processingtrajectory 526. Thus, for example, the series of dithered laser beamspots 534 at a given repetition rate has the effect of a series oflarger diameter laser beam spots successively applied in the directionof the processing trajectory 526 at a lower pulse repetition rate.

In certain embodiments, the AODs 530, 532 can update their respectiveacoustic fields (filling the optical aperture with a new acousticwaveform) on the order of about 0.1 μs to about 10 μs. Assuming anominal update rate of about 1 μs, the process beam's position can berapidly updated such that several of the dithered laser beam spots 534overlap during processing. The dithered laser beam spots 534 may overlapin the dimension (e.g., along the X-axis or dither direction)perpendicular to the processing trajectory 526 to widen the feature(e.g., the trench 528) being processed. As shown in FIG. 5, the ditheredlaser beam spots 534 may also overlap in the direction of the processingtrajectory 526. To keep the dithered beam oriented normal to theprocessing trajectory 526, according to certain embodiments, a ditheraxis may be constantly adjusted as the angle of the processingtrajectory 526 changes. In addition, the dither axis may be adjusted tocompensate for the angle imparted on the line of dither points as afunction of the process trajectory velocity. Given a trajectory velocityV, a dither update period Td, a number of dither points Npts, and adither excursion Dd, this angle equals atan[Td*(Npts−1)*V/Dd].

In addition to dithering the beam position with respect to the surfaceof the workpiece 524, or in other embodiments, the AOD subsystem 506 maybe used to vary the intensity profile in the dither axis. Themanipulation of the intensity profile of the process beam 512 along thedither axis enables shaping of the cross-section of the processed trench528. For example, the trench 528 may be processed with rectangular, U,or V shaped cross sections. Shaping features such as sidewall slope maybe useful in situations such as intersection formation. The shapingresolution may be based on the fundamental spot size, and the shapedintensity profile may be a convolution of the dither pattern (locationand intensity) and the spot intensity profile (e.g., Gaussian or anotherprofile shape). A feature may be shaped, for example, by overlappingpulses at certain locations (e.g., two or more pulses may be applied atthe same location) along the dither axis to remove selected amounts oftarget material, and/or by modulating the power amplitudes of laserpulses as a function of deflection position along the dither axis.

In addition to the shaping of the feature along the dither axis, or inother embodiments, the AOD subsystem 506 may be used to control thepower as a function of the position along the processing trajectory 526to allow similar shaping of the “endpoint” of a processed linearfeature. Controlling the power as a function of the position along theprocessing trajectory 526 may also be useful in applications such asintersection formation. The use of the AOD subsystem 506 enables powermodulation to occur at very high speeds (e.g., on the order ofmicroseconds) such that fine control of intensity profiles (e.g., withfeature dimensions in a range between about 5 μm and about 50 μm) may bepossible at high processing velocities (e.g., in a range between about 1m/s and about 5 m/s).

In addition to deflection of Gaussian beams, certain embodiments mayalso deflect beams shaped by traditional beam shaping technologiesincluding, for example, diffractive optical elements (DOEs). Forexample, FIG. 5A is a block diagram of a system 540 for beam shapingaccording to one embodiment. The system 540 includes the AOD subsystem506 (with the first AOD 530 and the second AOD 532), the zero order beamdump 516, and mirror 518 shown in FIG. 5. The system 540 furtherincludes a diffractive optical element (DOE) 542 for beam shaping andoptical elements 544 (e.g., imaging optics, galvo mirrors, and scanlens). For illustrative purposes, the first order beam 513 in FIG. 5A isshown over a range of AOD deflection angles 514. In embodimentillustrated in FIG. 5A, the first order beam 513 deflected by the AODsubsystem 506 is relayed to the DOE 542 through a relay lens 546(imaging the beam's pivot point on the DOE 542) to keep the first orderbeam 513 centered on the DOE's aperture regardless of the AOD deflectionangle 514 imparted by the AOD subsystem 506. The DOE 542 may then shapethe beam intensity by imparting additional wavefront phase distortion(as is typical for such beam shaping DOEs). This approach may bebeneficial in situations where larger, shaped beams may be deflected andabutted to form a more uniform dithered fluence profile with, forexample, square intensity profiles. This approach may also be beneficialin situations where a small number of laser pulses is adequate to form adesired feature (for example, a microvia drilled in a dielectricmaterial). In this case, a rastered application of Gaussian pulses maybe less efficient relative to applying a shaped intensity profile, yethigh-speed AOD deflection may be desirable for high-speed control of theshaped intensity process spot position.

In other embodiments, a similar relay lens configuration may be used totailor the deflection of the AOD-deflected beam at the scan lens. Thismay be desirable for at least two reasons. First, it may be desirable torelay the pivot point of the beam to the galvo scan mirrors (eliminatingbeam lateral deflection) to (a) keep the beam centered in the clearaperture of the galvo mirrors and scan lens to avoid beam clipping, and(b) avoid displacing the beam from the center of the scan lens entrancepupil, since such displacement may produce a sloped beam at theworksurface. Second, it may be desirable to impart a lateral beamdeflection at the scan lens in order to intentionally produce a beamslope at the work surface. A sloped beam may be advantageous in certainGaussian laser drilling applications to create a steeper sidewall inprocessed features (for example, microvia drilling).

FIG. 5B is a block diagram of a system 550 providing slanted processbeams 552 according to one embodiment. The system 550 includes the AODsubsystem 506 (with the first AOD 530 and the second AOD 532), the zeroorder beam dump 516, and mirror 518 shown in FIG. 5. The system 550further includes a relay lens 546 and optical elements 544 (e.g.,imaging optics, galvo mirrors, and scan lens). For illustrativepurposes, the first order beam 513 in FIG. 5B is shown over a range ofAOD deflection angles 514. As shown in FIG. 5B, by properly designingand spacing 554 the relay lens 546 from the scan lens (e.g., from thescan lens 520 shown in FIG. 5), the first order beam 513 deflected bythe AOD subsystem 506 can also be laterally deflected to create thesloped beam 552 at the surface of the workpiece 524. The amount of beamslope for a given deflection of the process spot at the workpiece 524may be controlled by either (a) using the AODs 530, 532 to substantiallycreate the lateral spot deflection at the workpiece 524, and varying therelay lens 546 optics and spacing 554 to the scan lens (e.g., scan lens520), or (b) coordinate the galvos (e.g., the galvos 533, 535 shown inFIG. 5) and AODs 530, 532 such that a arbitrary lateral beam deflectionat the scan lens (and thus an arbitrary beam slope at the workpiece 524)may be imparted independently from the desired lateral spot deflectionat the workpiece 524.

Further details of shaping techniques are disclosed below in the sectionlabeled “Example AOD Control Embodiments.”

While beam dithering may be very effective and flexible for producing adesired fluence profile, an alternative (but sometimes more restrictive)approach to dithering includes varying the focus of the laser beam spot522 by applying a chirp waveform to at least one of the AODs 530, 532.With a chirp waveform, the instantaneous frequency of the acoustic wavevaries linearly within the optical process beam 512 passing through theAOD's crystal. The linear variation of the instantaneous frequency ofthe acoustic wave has the effect of applying a single-axis (astigmatic)focusing term to the process beam 512, rather than displacing the laserbeam spot 522 in discrete steps. By applying chirp waveforms to bothAODs 530, 532, according to certain embodiments, the laser beam spot 522can be symmetrically defocused, thus increasing the spot size at theworkpiece 524. This approach may be useful, for example, in the case oflower repetition rate lasers where the pulse repetition frequency maynot be high enough to provide good overlap of pulses at the workpiece524 to avoid intensity variation when widening the trench 528.

IV. Rastering

Another operational mode that may be used with the AOD subsystem 506includes rastering a two-dimensional pattern very rapidly with the AODs530, 532. One use of rastering includes spatially shaping the intensityprofile of the process beam 512 to create desired features at theworkpiece 524, such as microvias. The AODs 530, 532 control both spotposition and intensity weighting, which enables the creation of anydesired intensity profile. In addition, it may be beneficial to vary thedwell time of the laser beam spot 522 at each location for processoperations using high intensity, such as copper drilling.

Using rastering provides several benefits over the traditional “shapedoptics” approach. Some benefits include, but are not limited to:arbitrary selection of process spot diameter (within the AOD range) orshape (e.g., round, square, rectangular, oval, or other shape); costreduction due to elimination of shaping optics, and/or Gaussian orshaped mode change optics; ability to process features with thehigh-intensity Gaussian beam (e.g., using spiral, trepan, or otherpatterns), at very high speed, without dynamic constraints due to galvobeam positioning; compensation for beam distortion (e.g., ellipticalspots) by modifying the raster pattern appropriately; and/or tailoringof the spatial intensity distribution on-the-fly to optimize or improvecharacteristics such as taper and/or bottom quality.

Several options are available to design the raster pattern (spotposition and amplitude). One embodiment includes filling in an area withan array of spot positions. This embodiment, however, may provide littlecontrol over the final cumulative fluence profile over the area. Forexample, the definition of the fluence profile at the edges of therastered area may not have the desired “slope” (e.g., change in fluencevs. position) for via formation or intersection processing.

In another embodiment, the fluence profile is explicitly defined and theraster pattern is selected to best fit the defined profile. This has anadvantage of creating customized fluence distributions, for example,with variable fluence levels throughout the raster area to vary depth,or with specifically shaped sidewalls at the edges of the raster area.This embodiment may be useful, for example, when combining fluences ofintersecting traces and/or when drilling vias with customized sidewalltaper.

FIG. 6 is a flowchart of a method 600 using a least-squares optimizationroutine to determine a set of spot amplitudes over a grid of rasterpoints according to one embodiment. As shown in FIG. 6, the method 600includes setting up 610 a candidate raster grid. For each spot in theraster grid, the method 600 includes calculating 612 a fluence profileover a raster field to generate a respective “influence function.” Themethod 600 further includes compiling 614 the influence functions intoan influence function matrix, calculating 616 the pseudo-inverse of theinfluence function matrix (e.g., using a singular value decomposition(SVD) algorithm), and calculating 618 the spot amplitudes at therespective grid points using the pseudo-inverse of the influencefunction matrix. The method 600 may further include applying 620 laserbeam spots to a workpiece according to the calculated spot amplitudes atthe respective raster points.

Example equations describing the method shown in FIG. 6 are outlinedbelow. The example equations assume a raster pattern, defined in XYcoordinates [xr, yr], containing Nr points. Applying a set of rasteramplitudes (Zr) produces a desired fluence surface (Zs), which may beevaluated at a set of XY coordinates [xe, ye] containing Ne points. Theinfluence matrix H is defined by:

Ze=H*Zr, is (Ne×Nr).

An operation to create the influence matrix H includes calculating thefluence of a single process spot located at one [xr, yr] point,evaluated over each [xe, ye] point. If the Zr and Ze matrices are“vectorized” for each evaluation, then Zr is (Nr×1) and Ze is (Ne×1).The procedure may be repeated for each [xr, yr] raster point, for atotal of Nr evaluations. Appending all results (Zr and Ze) into matricescreates a Zr diagonal matrix of size (Nr×Nr), and a Ze matrix of size(Ne×Nr). Normalizing the results by the applied amplitude of each spotresults in an identity matrix for Zr. The influence matrix H is then the(normalized) Ze matrix.

Given the influence matrix H, the desired actuator command vector Zrused to produce the desired surface fluence zDes may be given by:

Zs=Hinv*Zr.

Hinv can be calculated through an SVD decomposition, with the number ofmodes in Hinv limited to avoid excessive noise effects. Because theidentification of H (and Hinv) may be approximate, the calculation of Zrcan be performed in a closed loop mode, with a tuning gain applied:

Zr(k+1)=Zr(k)−kAlpha*Hinv*(zDes−Zs),

with Zs calculated in each iteration from a model or measured systemdata.

While the method 600 of FIG. 6 and the example equations described abovemay be fairly straightforward, the method 600 can be sensitive to theselection of the raster grid and the method of calculating thepseudo-inverse. However, the method 600 can provide an acceptableapproximation to the desired fluence profile within the limitations ofthe spatial characteristics of the fundamental laser beam process spot,which may impose a fundamental limit on the resolution at any edge ofthe raster pattern. FIG. 7A graphically represents a desired fluenceprofile and FIG. 7B graphically represents corresponding optimizedraster amplitudes determined according to the method 600 of FIG. 6 andthe example equations described above.

A related approach, according to another embodiment, involves optimizingthe raster pattern using a gradient descent method. In this embodiment,an objective function (e.g., the fit to a desired fluence profile) isdefined. The optimization process determines the local objectivefunction gradient (the incremental change in the function givenincremental changes in the fluence applied to each raster gridlocation), and uses the gradient in an algorithm to search for theoptimal vector of raster spot amplitudes.

Both approaches (SVD and gradient descent) can be performed either insimulation, or in a system. In the system, the actual fluencedistribution resulting from a given raster pattern (as measured by ametrology camera), can be used to quantify performance. Eitheroptimization method can then be applied to optimize the fluencedistribution, a process that avoids or reduces modeling errors andaccounts for tolerances in the system such as spot size and distortion,AOD linearization error, and/or optical alignment.

Of course, other optimization methods may be substituted for thealgorithms mentioned above.

V. Intersection Formation

Certain embodiments include the formation of intersecting processedfeatures (e.g., trenches, pads, vias, and other features), withcontrolled variation in the depth of the features at the intersection.For example, it may be desired to control electrical characteristicssuch as impedance (to maintain high-speed signal integrity), ordielectric breakdown (which may be sensitive to the gap between theplated trench and an underlying conductive layer), or to control platingquality.

Intersection processing can become quite difficult because the ablationof the workpiece dielectric may be proportional to the cumulativeapplied fluence when, for example, the process beam fluence is wellabove the ablation threshold of the material. In this situation,processing two intersecting features by simply crossing the featuresresults in “double exposure,” with close to 100% depth variation at thepoint of double exposure.

To avoid or reduce this problem, the system 500 discussed above withrespect to FIG. 5 may be used to “blend” the fluences of the twofeatures in the intersection region to reduce or avoid double exposure.For example, if the AOD subsystem 506 is used to process one trenchfeature with a wide “slope” of fluence at its sidewall, and to processthe intersecting trench feature with a matching fluence “slope” at itsendpoint, the two fluence distributions combine to produce a nominallyflat fluence field at the intersection. Thus, the AOD subsystem 506 maybe used to create a depth-controlled intersection.

The creation of a fluence slope provides other benefits, such as aminimization of depth variation due to beam positioning tolerances. Witha steep fluence slope in the intersection region, small variations inbeam position (e.g., on the order of about 1 μm to about 5 μm) duringthe ablation of the intersecting features can cause significant depthvariation. By creating a gradual slope in fluence, the beam positioningerrors produce acceptable depth variation (e.g., less than about 10% toabout 15% of the nominal depth).

The creation of a fluence slope may be implemented as the intersectingfeatures are processed by changing the fluence slopes and/or widthson-the-fly. In another embodiment, the trench features are terminatedoutside of the intersection (with appropriate fluence slopes), followedby a rastering of the remainder of the intersection volume. Thisembodiment has several advantages including, for example: lesssensitivity to the type of intersection (e.g., angles of theintersecting trenches, multiple trenches intersecting at a single point,intersections of curved trenches); minimized extra line width used tocreate side fluence slopes, which can force undesirable variations inthe spacing between adjacent trenches; and/or the ability to customizethe raster pattern to optimize characteristics of the intersection.Customizing the raster pattern may be useful when processing arbitraryshapes at the intersection, such as circular pads with multipleintersecting traces.

Additional details of intersection processing are disclosed herein inthe section labeled “Example Intersection Processing Embodiments.”

VI. Galvo Error Correction

One source of repeatability error (which may limit the ability tomachine intersections with good depth control, as described above) maybe the positioning error of the galvo subsystem 508 shown in FIG. 5.These errors may be due to sensor noise and tracking error. Each galvomirror 533, 535 in the galvo subsystem 508 may be associated with afeedback sensor (not shown) to control mirror movements using respectivegalvo servos (not shown). Sensor noise effects can occur when the galvoservo tracks the feedback sensor noise within the servo's bandwidth,resulting in physical beam motion. This error excitation may also beamplified by the closed-loop response of the galvo, which amplifies someportion of the frequency spectrum. Sensor noise effects may produce beamerrors, for example, from about 0.1 μm root-mean-squared (RMS) to about5 μm RMS, depending on the particular optical and servo design.

The sensor noise effects may occur at all times, regardless of thecommanded beam trajectory. Tracking error, however, occurs when thegalvos are commanded to follow a dynamically aggressive beam trajectory(containing large acceleration or high-frequency commands). Theinability of the galvo servo to track the commands leads to a trackingerror and a resulting loss of repeatability. Tracking errors may be theresult of, for example, linear servo response performance and/ornon-linear galvo behavior (such as bearing friction or magnetichysteresis).

To reduce both the sensor noise error and tracking error sources,according to one embodiment, the deflection capabilities of the AODsubsystem 506 are used to correct for galvo errors, as indicated by theposition sensor feedback. The sensor reading includes sensor noise,which may be filtered out above a reasonable bandwidth in order toreduce or avoid adding undesirable beam motion in response to thisnoise. In one embodiment, filtering substantially matches both phase andgain of the transfer function between galvo position sensor and beamdisplacement (the “beam transfer function”, or BTF) within a bandwidthof interest, while filtering higher frequencies. In certain embodiments,the BTF is strongly influenced by dynamics between the position sensorand the galvo mirror, often well modeled by a lightly damped secondorder pole. Other factors that affect phase matching, such as the timedelays due to signal filtering and data communication, may be includedin the design of the error correction filter. FIG. 8 graphicallyillustrates one example embodiment of an AOD error correction filterthat provides a compromise between the conflicting requirements of phaseand gain matching below about 10 kHz, and sensor noise filtering aboveabout 10 kHz.

Sensor noise rejection may also be accomplished through alternativessuch as estimation (e.g., Kalman filtering), at the risk of reducedperformance in some embodiments due to unmodeled dynamics or non-linearbehavior.

VII. PSD Mirror Sensing for Beam Positioning Accuracy Improvement

In certain embodiments, AOD error correction is enhanced using externalsensors that detect the actual galvo mirror position. In certaingalvo-based beam positioning systems, an angular position sensor isbuilt into the galvo to sense mirror angle. Sensors can be located atthe far end of the galvo shaft (away from the mirror), while others arelocated at the shaft end near the mirror.

When the angular position sensor is located at the far end of the galvoshaft away from the mirror, the sensor detects shaft rotation. However,shaft angular deflection can cause the mirror to have a differentdeflection angle. This sensor placement has some advantages in providingthe ability to increase servo loop bandwidth because it does not respondto the mirror resonance.

When the angular position sensor is located at the shaft end near themirror, the sensor detects angular deflection of the shaft nearer to themirror. This sensor placement more accurately measures the true mirrorangle. However, the sensor may still be subject to error when the mirroritself flexes relative to the shaft at the sensor. In addition, withthis sensor placement the resonance of the shaft and mirror appear inthe galvo frequency response (from motor drive to sensor output),complicating the galvo servo design and limiting its performance.

In addition, neither sensor placement can measure mirror modes notrelated to shaft angle. One mode includes that of a “flapping” mirror,in which the mirror plane rotates about an axis perpendicular to therotating shaft. This mode may be a limitation on the performance ofhigh-speed galvo deflection systems.

A further issue with galvo rotational sensors includes their noiseperformance. Due to the small package size of the galvos, and a desireto minimize the size (and rotational inertia) of the position sensors,the electrical noise present in the sensor circuitry translates to asignificant effective angular noise, which may degrade galvo servopositioning performance. For example, this noise may equate to about 0.1microradian (μRad) RMS to about 5 μRad RMS within a 10 kHz bandwidth.

In one embodiment, a different sensor may be selected that: detects thetrue angular position of the mirrors, without shaft deflection effects;detects all modes of mirror motion that affect beam position accuracy;and produces an angular measurement with low noise levels, such that themeasurement can be used by the galvo servo loop, or other devices, tocorrect for the sensed error.

To correct for errors in the actual galvo mirror position, according tocertain embodiments, the galvo subsystem 508 shown in FIG. 5 includes anauxiliary sensor (not shown in FIG. 5) that provides feedback to holdthe galvo mirrors in relation to the scan lens 520. For example, FIG. 9is a block diagram of a laser processing system 900 including anauxiliary sensor 910 in a galvo subsystem 912 according to oneembodiment. The auxiliary sensor 910 in this example includes a positionsensing diode (PSD) and is referred to herein as the PSD 910. The laserprocessing system 900 also includes an AOD subsystem 506, a scan lens520, a reference beam source 914, and a reference combiner mirror 916.The AOD subsystem 506 and scan lens 520 are described above withreference to FIG. 5 for providing a focused process beam 922 to asurface of a workpiece 524.

The reference combiner mirror 916 combines a reference beam 918 from thereference beam source 914 and a process beam 920 from the AOD subsystem506 for input to the galvo subsystem 912. The reference beam combiner916 may comprise, for example, a dichroic mirror, a polarizingbeam-splitting mirror, or other similar device for combining laser beamsto provide a reference beam with stable power and pointing angle. Theuse of the processing beam 920 from the AOD subsystem 506 may bepossible during PSD sensing operation (but not required), if theprocessing beam's position and power are sufficiently stable for theparticular application.

In addition to the PSD 910, the galvo subsystem 912 includes galvomirrors 924, a PSD pickoff mirror 926, and a PSD lens 928. The referencebeam 918 reflects off the galvo mirrors 924 (along with the mainprocessing beam 920). The PSD pickoff mirror (e.g., a splitting mirror)picks off a deflected reference beam (e.g., deflected by the galvomirrors 924), and directs the deflected reference beam to the PSD fordetection.

The PSD lens 928 (e.g., a focusing lens) may optionally be inserted inthe measurement path such that only angular motion of the deflectedreference beam is translated to XY spot deflection on the PSD 910, withlateral beam motion converted to a beam angle at the PSD 910, and thusnot measured in the PSD XY plane. In certain embodiments, the PSD lens928 includes a compact telephoto lens with a long effective focallength, to amplify the spot motion at the PSD plane. In certain suchembodiments, the PSD lens 928 is located such that the front focal pointis located at the scan lens entrance pupil. It may be possible to omitthe PSD lens 928, in certain embodiments, if lateral beam motion is nota concern, and the scaling of the beam motion on the PSD 910 is adequatefor the particular application.

The independent reference beam 918 and the PSD 910 can be selected suchthat the beam power, in combination with the PSD optical sensitivity,provide adequately low noise. A dominant noise source in PSDmeasurements may be “shot noise,” or the noise produced by quantizationof charge carriers (individual electrons) in the output current. Thesignal-to-noise ratio (SNR) may be proportional to the square root ofcurrent. By raising the output current to a high level, the SNR may beimproved, and low-noise angle measurement may be possible.

Once the PSD sensing is in place, the output of the PSD 910 vs. theposition of the focused process beam 922 at the workpiece can be easilycalibrated. Given a calibrated PSD 910, beam positioning can be improvedin several ways. For example, the PSD 910 can be used as the positionfeedback sensor for the galvo servos. This may be complicated by thefact that it creates a cross-coupled system, complicating the dynamicsof the feedback system.

In addition, the non-rotational mirror modes (“flapping” and othermodes) may be difficult to accommodate in the servo loop. Dynamicestimators (e.g., Kalman filters, Luenberger observers, or otherestimators) can be used to separate dynamic modes and improve the servoloop design.

In addition, or in other embodiments, the PSD 910 can be used forlimited error correction by the galvo subsystem 912 itself. For example,mirror cross-axis modes can be corrected by the galvos, if theirfrequency content is within the galvo servo bandwidth. In addition,low-frequency noise error in the built-in galvo sensors (not shown) canbe rejected by blending feedback from the PSD 910 (at low frequency) andthe built-in sensor (at higher frequencies).

In addition, or in other embodiments, the PSD position reading can beused for open-loop error correction by a separate device, such as an AODincluded in the beam path. This may be a useful operational mode becauseit separates the galvo dynamics from the error correction system. Usinga reference beam separate from the main process beam (which is deflectedby the AOD subsystem 506, and thus sensed by the PSD 910) allows the AODerror correction to operate in an “open loop” mode, in which the AODerror correction does not affect the PSD beam position output. This cansimplify the error correction algorithm significantly. Both noise andmirror deflection modes are easily corrected in such an embodiment.

If the process beam 920 is also used as the PSD reference beam 918,similar AOD error correction may still be possible, with the AODsubsystem 506 forming a closed error correction loop. In this case, thePSD reading is analyzed to remove any intentional AOD deflectioncommands (such as dithering, rastering, and/or high-dynamic beampositioning) because intentional commands are sensed by the PSD 910.

In certain embodiments, it may be useful to include a first PSD sensorthat senses a separate reference beam angle, and a second PSD (notshown) that senses the process beam angle, to combine the benefits ofthe above embodiments. For example, a second PSD may be used fordiagnostic measurement and process quality monitoring.

VIII. Throughput Improvements: AOD/Galvo Coordination and TertiaryProfiling

Certain laser processing applications, such as laser direct ablation(LDA), ablate features at high process beam velocities (e.g., velocitiesin a range between about 0.2 m/s and about 5 m/s) to achieve highthroughput. One challenge of implementing high-velocity processing maybe the dynamic limitations of the galvo beam positioning systems used tocontrol the process beam position. During processing of some features,such as short arc segments, the beam positioner accelerates to changethe beam velocity trajectory. LDA applications, for example, may requireprocessing of a feature with a tight turn radius on the order of tens ofmicrons or less with a desired repeatability (within the galvo field) ofabout 1 μm. FIG. 10 is a schematic illustrating example trench patternsprocessed for an LDA application according to certain embodiments. Theembodiments disclosed herein provide for high speed processing of trenchintersections 1010, pad intersections 1012, pad intersections with tighttransitions 1014, and other features associated with LDA processing.

As process beam velocity increases or arc segments become shorter, theacceleration occurs over shorter time periods during which the beampositioner uses higher bandwidth control. This may eventually become alimitation in the ability to reach high velocities.

Referring again to FIG. 5, this limitation can be avoided by performingthe high-bandwidth portion of beam trajectory control using a high speeddeflector such as the AOD subsystem 506. In this approach, the galvotrajectory can be designed to approximately follow the desiredprocessing trajectory 526, while remaining within the dynamicconstraints (e.g., acceleration and/or bandwidth) of the galvo subsystem508. For example, FIG. 11 graphically represents curves associated withAOD and galvo coordination according to one embodiment. As shown in FIG.11, the galvo path 1110 may not be able to precisely follow a desiredprocess path 1112, resulting in a beam trajectory error. The beamtrajectory error can be removed by using the AOD subsystem 506 toadditionally deflect the process beam 512. For example, FIG. 11 shows anAOD command signal 1114 in relation to the galvo path 1110 and processpath 1112. Because the two trajectories (galvo and process beam) areknown in advance, the AOD deflection trajectory can be calculated andverified that it satisfies the AOD's constraints (e.g., range andmaximum normalized power over this range). The galvo trajectory can betailored such that the residual error does not violate the AOD'sconstraints. This may be an iterative process; for example, the galvovelocity may be reduced in certain portions of the tool path such thatthe galvo can more closely track the selected trajectory and thus keepthe resulting trajectory error within the AOD range limits.

Another embodiment includes “passively” correcting for galvo trackingerror using the AOD subsystem 506, as described above. In thisembodiment, the selected processing trajectory 526 is planned withoutexplicit constraints, and the galvo subsystem 508 attempts to followthis path, with any resulting tracking error corrected with the AODsubsystem 506. Limitations to this approach may include excitation ofundesired dynamics in the galvo (e.g., mirror resonances) and the riskthat the performance of AOD error correction is not sufficientlyadequate to reduce tracking error so as to meet the overall beampositioning requirements for a particular application.

In certain embodiments, AOD correction of galvo error can be used withseparately generated beam and galvo trajectories to remove residualgalvo tracking error. The AOD operation described in any of the aboveembodiments can be applied while simultaneously dithering or defocusingthe beam to control effective spot size.

Another limitation to throughput can be the laser power available toprocess different portions of a particular pattern. For a givenmaterial, the process dosage (power divided by velocity) may be afunction of the cross-sectional area of the ablated feature. If thesystem is using the maximum available laser power, the process beamvelocity may be determined by dosage (velocity=power/dosage). It may bedesirable to maintain the highest possible velocity, despite changes indosage, to maintain high throughput. In some cases, this may bedifficult due to rapid changes in the dosage. For example, it may bedifficult to obtain a high velocity when a relatively thin trench widensby about five times (5×) to form a large-area feature. In this case, ifthe beam velocity remained constant over the length of this feature set,the velocity may be constrained by the high dosage used at the expandedarea. This may unnecessarily slow down the beam velocity along thethinner trench. Rapid acceleration and deceleration of the main beampositioner (galvos) may be undesirable, especially for rapidtransitions, due to the dynamic limitations of the galvos.

To avoid or reduce rapid acceleration and deceleration of the main beampositioner, according to one embodiment, the available AOD fieldproduces a lower beam velocity over a short segment. For example, assumea trench feature may be processed at a velocity of about 2 m/s, and theexpanded feature, with a length of 100 μm, may be processed three times(3×) slower (due to a 3× higher specified dosage). If the galvo velocityof 2 m/s is unchanged, the beam nominally passes over the wide featurein about 50 μs. However, the beam processes the feature at a beamvelocity of about 2/3=0.67 m/s (to maintain proper dosage). Thus therelative velocity of the AOD deflection is about 2−0.67=1.33 m/s, whichmay be applied for about 50 μs, resulting in an AOD deflection of about67 μm (+/−about 33 μm). By avoiding velocity limitations in the entiresection due to the wide features, this example embodiment effectivelyincreases the local process speed by about 3×. FIG. 12, for example,illustrates the velocity trajectories during such a period. FIG. 12graphically represents curves associated with AOD velocity compensationat a wide feature or other feature that uses a high dosage. Inanticipation of the wide feature, the beam velocity speeds up locallyand the AOD position shifts. Upon reaching the wide feature, the beamvelocity is reduced and the AOD position slews across its field tocompensate for the slow down. After processing the wide feature, thebeam velocity recovers its speed and the AOD position returns to itsneutral position.

In addition, or in another embodiment, the mean velocity of the beam canbe dynamically varied (modulated) while maintaining constant dosage(power/velocity). In certain such embodiments, the AOD subsystem 506modulates the process beam power as a function of the instantaneousgalvo velocity. The power modulation provides the capability to slowdown the galvo mirrors 533, 535 to process sections of the workpiece 524with more restrictive dynamic requirements (e.g., rapid and frequentchanges in the orientation of the ablated feature) or speed up the galvomirrors 533, 535 in sections with relaxed dynamic requirements (e.g.,straight sections or sections with very gradual changes in orientation).Without real-time AOD power control, such capabilities are not possible,leading to loss of throughput.

Another opportunity for throughput improvement occurs when processingparallel line segments (e.g., adjacent trenches). In certain suchembodiments, the AOD subsystem 506 can toggle the process beam 512between the two lines, at a sufficiently fast rate (e.g., around 1 μs)such that the two lines appear to be processed simultaneously. For twolines of equal dimension, requiring an equal dosage, parallel processingdoubles the required laser power. In a power-limited system, this mayrequire the velocity to be reduced by about 50%, limiting the throughputadvantage. However, it may eliminate setup moves for the two lines andreduce the velocity-dependent dynamic constraints that may otherwiseprevent the use of full velocity. In systems without such power limits,parallel line processing may double throughput for that section. Inaddition, in both cases, parallel processing of such lines improvesline-line spacing control (which is controlled by the repeatability ofthe AOD subsystem 506), which is a benefit in some applications tocontrol line impedance.

When the lines turn a corner, the path length of the two lines differdepending on the turn radius, turn angle, and line separation. Incertain such embodiments, the effective beam velocities of the two linescan be adjusted to account for the different path lengths of the twoline segments. The power (dosage*velocity) may then be modulated by theAOD subsystem 506 as processing switches between the two lines. The sameapproach can be extended to multiple lines, as long as the line set fitswithin the available AOD field and sufficient process beam power isavailable to produce an adequately high velocity.

Processing multiple lines at the same time may be complicated when themultiple lines do not remain parallel over their entire respectivepaths. In certain such embodiments, the appropriate region can beprocessed in parallel, and a transition region applied to the end of theregions (e.g., sloped fluence), as in the technique used forintersection processing. The remaining section can then be processed andjoined at the termination with a similarly transitioned region. In thisway, the use of parallel processing can be maximized to enhancethroughput.

FIG. 13 schematically represents parallel processing and region joiningaccording to one embodiment. FIG. 13 illustrates a plurality of lines1306(a), 1306(b), 1306(c), 1306(d) (referred to collectively as lines1306) that run parallel to one another in a first region 1308 anddiverge from one another in a second region 1309. In other words, thelines 1306 have parallel portions in the first region 1308 and divergingportions in the second region 1309. FIG. 13 illustrates a transitionregion 1310 where the lines 1306 change from the parallel portions tothe diverging portions.

To process the lines 1306 using dithering in the first region 1308, theAOD subsystem 506 moves the laser beam spot position back and forthbetween the parallel portions of the lines 1306 such that the laser beamprocesses the parallel portions in a single pass along the mutualprocessing trajectory in the first region 1308. The AOD subsystem 506adjusts the effective beam processing velocities between the parallelportions to account for turns along the processing trajectory thatresult in different path lengths corresponding to the respectiveparallel portions. The AOD subsystem 506 also modulates the power of thelaser beam based on the adjusted effective beam processing velocities tomaintain desired processing dosages for each of the parallel portions.Upon arriving at the transition region 1310, the AOD subsystem 506selectively shapes the parallel portions of three of the lines 1306(a),1306(b), 1306(c) and, while continuing to process the line 1306(d) alongits processing trajectory, terminates processing of the three shapedlines 1306(a), 1306(b), 1306(c) in the transition region 1310. Theshaping allows the terminated lines 1306(a) 1306(b), 1306(c) to beintersected (by their respective diverging portions), at a later pointin time, while maintaining a desired depth at the respectiveintersections. After processing the diverging portion of the line1306(d), the diverging portions of the lines 1306(a), 1306(b, 1306(c)are sequentially processed along their respective diverging processingtrajectories in the second region 1309.

FIG. 14 schematically illustrates a tertiary profiling subsystem 1400according to one embodiment. Tertiary profiling refers to using the AODsubsystem 506 as a tertiary positioner (e.g., in addition to XY stagesand the galvo subsystem 508). An example laser beam tertiary positioneris described in U.S. Pat. No. 6,706,999, which is assigned to theassignee of the present disclosure, and which is hereby incorporated byreference herein in its entirety. Tertiary profiling using the AODsubsystem 506, as disclosed herein, allows for profiling the beam pathat high speed (e.g., using updates at about 1 μs to provide timingresolution) wherein AOD commands are issued on discrete timingboundaries. The tertiary profiling subsystem 1400 includes a profilingfilter 1404, delay element 1406, and a subtractor 1408.

FIG. 14 illustrates an example beam profile 1410 corresponding to atrench that is desired to be cut into a workpiece. The example beamprofile 1410 includes sharp turns that may be difficult to track at highvelocities using the galvo subsystem 508. The example beam profile 1410is provided (as a commanded beam position signal) to the profilingfilter 1404 and the delay element 1406. The profiling filter 1404comprises a low-pass filter that filters out high frequency content thatmay be difficult for the galvo subsystem 508 to track. The output of theprofiling filter 1404 may be used as a galvo command (galvo controlsignal), as shown by position profile 1412. FIG. 14 illustrates anenlarged portion 1413 of the position profile 1412, which shows acommanded position 1416 with respect to an actual position 1418 providedby the galvo subsystem 508. The AOD subsystem 506 is used to correct forthe difference between the commanded position 1416 and the actualposition 1418.

In the illustrated embodiment, the profiling filter 1404 comprises afinite impulse response (FIR) filter. FIR filters naturally have aconstant delay for signals in any frequency range. An artisan willrecognize from the disclosure herein, however, that other types offilters may also be used. The delay element 1406 delays the example beamprofile 1410 by approximately the same amount of delay introduced by theprofiling filter 1404. The subtractor 1408 subtracts the output of theprofiling filter 1404 from the output of the delay element 1406 to getthe high frequency content that was removed from the galvo command. Thehigh frequency content output by the subtractor 1408 may then be used asan AOD command signal for controlling the AOD subsystem 506. FIG. 14illustrates an example AOD position command profile 1414. Although notshown, differentials may be used on the position command profile 1414 tocalculate corresponding velocity and acceleration command profiles.

By way of further example, and not by limitation, FIGS. 15A, 15B, 15C,15D, and 15E illustrate signals produced and/or used by the tertiaryprofiling subsystem 1400 shown in FIG. 14 according to one embodiment.FIG. 15A illustrates an example beam profile input to the tertiaryprofiling subsystem 1400. FIG. 15B graphically illustrates X, Y, and XYbeam velocity trajectories corresponding to the example beam profile ofFIG. 15A. FIG. 15C graphically illustrates X and Y beam accelerationtrajectories corresponding to the example beam profile of FIG. 15A. FIG.15D illustrates example galvo dynamics, including commanded position,velocity, and acceleration signals. FIG. 15E illustrates exampletertiary (AOD) dynamics, including commanded excursion and errorsignals.

FIGS. 16A, 16B, and 16C illustrate example AOD command sequencesaccording to certain embodiments. In FIG. 16A, the AOD command sequenceupdates the AOD at about 5 μs intervals, which are aligned to 5 μsboundaries. Such an embodiment may be sufficient for low speedprocessing (e.g., processing at about 200 mm/s). In FIG. 16B, a highspeed AOD command sequence updates an AOD at approximately 1 μsintervals that are not aligned to timing boundaries. Arbitrary timing(with updates unaligned to any timing boundaries) may be difficult toimplement in some embodiments. For example, intersection processing mayrequire that AOD transitions occur with about 1 μs resolution, at a highspeed of about 2 m/s to about 3 m/s. To satisfy such requirements,arbitrary layouts processed at constant velocity may use variable,sub-microsecond timing resolution. Further, variable velocity processingmay be desirable to: provide high velocity for narrow trenches forincreased throughput; provide lower velocity for wide trenches tomaintain fluence; to slow down (or pause) over rastered areas to rasterinline (e.g., to reduce or eliminate return moves); and/or to slow downif necessary to improve galvo tracking and to keep the AOD within itsbounds.

In one embodiment, timing is provided by position-based AOD commands andan arbitrary beam trajectory. In such an embodiment, triggering is basedon XY position. Thus, for a non-linear beam positioner trajectory, thetrigger is based on the X axis or Y axis, depending on the featurelocation. This embodiment may have increased field programmable array(FPGA) data processing requirements due to streaming first-in-first-out(FIFO) trigger data as the workpiece is processed, realtime positioncommand data (both X and Y), and wide dynamic range used duringprocessing (e.g., high resolution over full field). This embodiment mayalso use command triggering that does not provide for variable velocityprocessing.

In another embodiment, timing is provided by time-based AOD commands anda modified beam trajectory. This embodiment may limit AOD commandtransitions to segment boundaries, wherein segments may be subdivided asrequired to include AOD commands. This embodiment aligns segments toposition (not time) boundaries, which uncouples processing geometry frombeam velocity. This embodiment also adjusts segment velocities such thatboundaries are hit at regular time intervals, which provides: flexible,variable process velocity; velocity variation that is easily implementedwith the tertiary beam positioner; and reasonable restrictions on AODtransition spacing to constrain velocity variation. This approach may beadvantageous, for example, in pulsed laser systems, which may requirelaser pulsing control on discrete, predictable time boundaries.

In FIG. 16C, AOD updates occur at 1 μs intervals and are aligned todiscrete, deterministic 1 μs boundaries. Feature positions, however, maybe arbitrarily located in the toolpath. To align arbitrarily locatedfeatures in time, the velocity between features is predetermined. Thus,in certain such embodiments, velocity modulation is used from segment tosegment between features. FIGS. 17A and 17B graphically illustrateexamples of velocity modulation according to certain embodiments. InFIG. 17A, the velocity modulation uses timing based on the positionincrements corresponding to an optimum process velocity. In FIG. 17B,the velocity modulation adjusts velocity by aligning timing to discretetime increments (same position increments). In certain embodiments, atime delta (A) between segment boundaries, nominally equal to Dseg/Vnom(where Dseg=segment length and Vnom=max process velocity) is rounded upto a next discrete timing boundary. During velocity modulation, thedosage may be held constant (e.g., by varying power as velocitychanges). A minimum position increment (ΔPmin) and a maximum timingincrement (ΔT) may be selected so as to bound a relative drop in processvelocity to Vnom*ΔT/ΔPmin, where Vnom is a nominal velocity. Forexample, ΔT may be selected to be 1 μs and ΔPmin may be selected to be20 μm to provide a 15% velocity variation at 3 m/s. Because the velocityvariation occurs for only a short period of time, it may have little orno effect on throughput.

Tertiary profiling may produce a positioning error that results from adiscrete AOD update period and an AOD velocity term. For example, FIG.18 graphically illustrates a positioning error 1810 with respect to aposition command signal 1812 and a resulting AOD position profile 1814according to one embodiment. As shown in FIG. 18, the AOD does not slewbetween command updates in the same manner as would, for example, amechanical mirror. Thus, for example, a 1 μs update period at a velocityof 3 m/s may produce a positioning error of about +/−1.5 μm. The updateperiod may be reduced if required to limit this error. Chirping the RFwaveform may reduce positioning errors in certain embodiments. FIG. 15Egraphically illustrates an example position error.

IX. Beam Pointing Stabilization

In certain laser processing applications that provide good accuracyand/or repeatability, the contribution of angular or translational beamjitter can become a significant portion of the error budget. Jitter maybe due to beam motion inherent to the laser source, or due to airturbulence in the beam path (exacerbated by air temperaturedifferentials within the beam path), and/or mechanical vibrations withinthe optics train. In a laser scanning system, angular errors contributedirectly to position errors at the workpiece, when scaled by the scanlens focal length. Beam translational errors indirectly contribute toworkpiece errors by producing an (uncompensated) beam angle at theworkpiece; this angle, scaled by any Z height variation of the workpiecesurface, produces XY beam positioning errors on the workpiece.

A system equipped with AOD deflection capabilities, such as the system500 shown in FIG. 5, can correct for beam jitter, with little or noadditional actuation cost. By placing a sensor (not shown) along theoptical path (e.g., near the scan lens 520), feedback control cancommand an AOD deflection so as to keep the beam correctly positionedinto the scan lens 520, which may improve beam position accuracy andrepeatability. The frequency content of many beam jitter sources may berelatively low (e.g., less than about 10 Hz for air turbulence and lessthan about 500 Hz for mechanical vibration) and thus easily correctableby the AOD subsystem 506. A limiting factor in this approach may be thenoise content of the sensor used to detect beam angle and translation.

The intentional deflection of the AOD (commanded to produce a selectedworkpiece trajectory) may be accounted for when making measurements. Forexample, in an optical train in which relay optics are not used totransmit the AOD-deflected beam to the scan lens 520, the AOD'sdeflection angle, when scaled by the beam path length from the AODsubsystem 506 to the scan lens 520, produces a translational offset atthe scan lens 520. A simple calibration allows this to be removed fromthe measurement before jitter correction. The calibration can beperformed as a function of path length from the AOD subsystem 506 to thescan lens 520, if necessary. In general, however, if the process beam isfocused on the sensor, no such compensation may be required becauselateral beam motion will not affect spot position at the sensor.

Note also that jitter correction can correct for undesirable sideeffects from AOD operation, such as thermal drift of the beam due to AODheating, which may occur for high-powered AOD devices.

X. Process Improvements: Duty Cycle

In some embodiments, the AOD subsystem 506 enables laser/materialinteraction process improvements. In one example, the cross-sectionalarea of a trench cut into a dielectric material is sensitive to the“dosage” (process beam power divided by beam velocity) applied to theworkpiece. For best or improved performance in some applications, theapplied dosage may be kept much higher than the ablation threshold ofthe material in order to avoid heat affected zone (HAZ) effects, such asmelting or charring. At low velocities, which may be used in somesituations due to constraints imposed by beam positioner dynamics orlaser power, the applied dosage may begin to produce undesirable HAZeffects. Thus, to avoid or reduce HAZ effects according to oneembodiment, the AOD subsystem 506 modulates the power duty cycle of theprocess beam 512, such that high peak power is maintained while reducingthe average power to the level of that used for the particular operatingcondition. For example, when moving the process beam 512 at about 100mm/s, modulating the beam with a duty cycle of about 10% (about 1 μs on,9 μs off) produces an acceptably small “bite size” (incremental processlength per pulse interval) of about 1 μm, while increasing peak power byabout ten times (10×) relative to an attenuated 100% duty cycle beam. Aswith the jitter correction outlined above, this capability may be addedwith little or no additional cost.

XI. Process Improvements: Plume Avoidance

AOD operation may also provide the capability to reduce or avoid plumeeffects during ablation of target material on or within the workpiece.Material ablated from a workpiece, which may be ejected as a plasma,gas, or particulate debris, can affect the quality of the process beamspot through, for example, wavefront distortion, power attenuation,and/or pointing effects. To mitigate plume effects according to oneembodiment, the position of the process spot is switched duringprocessing such that each spot is unaffected by the plume effects of theprevious spot. If the process spot position can be switched between Npositions (with all spots lying along the selected process trajectory)over the available AOD field distance (Daod), when running at a processvelocity V, the plume of the forward process spot dissipates forDaod/V/N seconds before it affects the next spot. For example, when theposition N=5, Daod=50 μm, and V=2 m/s, the plume of the forward processspot may have about 5 μs to dissipate before it affects the next spot.When the process trajectory includes curved segments, the positions ofthe distributed spots may be adjusted to remain on the selectedtrajectory.

XII. Through-the-Lens Viewing

In laser processing machines, the processing beams are aligned toworkpiece features. The alignment may be performed by identifying aworkpiece alignment fiducial (e.g., an alignment target) with analignment camera, and then mapping the camera field of view to theprocess beam position through calibration. Speed and efficiency may bereduced because this uses a two-step process (involving laser-to-cameracalibration error, and camera fiducial identification error), andbecause the camera and scan lens are separated from each other, whichadds another uncertainty due to positioning stage repeatability andaccuracy.

In certain embodiments, it is more desirable to use through-the-lensviewing to capture both the process beam and the workpiece view in oneimage, which enables measurement of the relative position between thebeam and a fiducial target. This may be accomplished by designing thescan lens 520 shown in FIG. 5 to operate at two wavelengths (a processwavelength and a vision wavelength). Compromises may sometimes be madein the scan lens design in order to accommodate both the processwavelength and the vision wavelength. One embodiment overcomes thiscompromise by illuminating the workpiece 524 with the process beamwavelength when imaging the fiducial.

For example, FIG. 19 is a block diagram of a system 1900 forthrough-the-lens viewing using an AOD subsystem 506 for rasterillumination according to one embodiment. The system 1900 also includesa camera 1910, an imaging lens 1912, a mirror 1914, a partial reflector1916, and a scan lens 520. As discussed above with respect to FIG. 5,the AOD subsystem 506 includes AODs 530, 532 for deflecting a processbeam 512 provided to the scan lens 520, which provides a focused laserbeam 525 to a workpiece 524. It may be difficult to find an illuminationsource with the same wavelength as the process beam 512, and using adifferent wavelength (even within a few nanometers) may degrade theinspection resolution.

Instead, a portion of the process beam 512 can be split off and used toilluminate the workpiece 524. This can be performed with beam splittersand diffusers or other defocusing elements, which may add additionaloptical components, alignment, and complexity. Thus, in the embodimentshown in FIG. 19, the AOD subsystem 506 is used to illuminate theworkpiece 524 using a raster pattern that fills a region on theworkpiece 524 with uniform fluence, and includes one reference spot ofhigher intensity to use as a reference for the alignment of the processbeam to the workpiece 524. The partial reflector 1916 picks off lightreflected from the workpiece 524, the mirror 1914 redirects thereflected light through the imaging lens 1912 to focus the image at thecamera 1910. Adding the optical path to pick off and re-image lightreflected off the workpiece 524 into the scan lens 520 allows the camera1910 to image the workpiece 524 and reference spot and determine therelative alignment of beam to fiducial.

The embodiment shown in FIG. 19 can also provide the capability ofdetermining the correct scan lens Z height adjustment to maintain spotfocus (minimizing the spot size in the alignment camera). Recording there-imaged spot size at three to five separate lens Z heights may provideenough information to derive the optimum spot position after a curve fitto expected spot size growth vs. Z height.

This alignment technique can provide very good beam-to-fiducialalignment accuracy (e.g., less than about 10% of the spot size). Thiscan provide the capability to quickly align two separate scan fieldareas on a workpiece because the XY position on the workpiece may be thesame for processing and alignment. By aligning the spot of one scanfield to a processed feature of an adjacent scan field, the two processfields may be separately processed yet joined together with highaccuracy.

An additional feature of through-the-lens viewing is the ability to mapbeam positioning errors over the surface of the part to be processed.One challenge of processing a part over such a large field is that thebeam may be non-telecentric (e.g., the beam angle at the workpiece maybe non-zero or off normal), and varying over the field. This angle,multiplied by any Z height variation, produces an XY positioning error.Telecentric angles for large-field scan lenses may be up to about 15degrees. Workpiece surface variation may be about +/−100 μm. Thus, thecombination of telecentric angles and workpiece surface variation mayproduce XY errors of up to about +/−26 μm (relative to, e.g., an errorbudget in a range between about 10 μm and about 20 μm). By usingthrough-the-lens viewing to record these position offsets at severalpoints in the field (enough samples to accurately map the Z terrain ofthe part over the process field), the telecentric errors can be removed.A set of reference fiducials on the workpiece may be used such that thelocation of the process spot, relative to the fiducials, may beaccurately measured (to within a few microns). Absolute measurement ofspot position in a through-the-lens setup may be difficult. Spotposition measurements, however, may be performed during alignment or inreal-time during part processing.

XIII Example AOD Control Embodiments

(a) Introduction

This section describes LDA processing using AOD control architectureaccording to one embodiment, including example equations that determinethe AOD deflection commands during processing.

The AOD provides at least two functions: (1) beam position dithering toexpand the process beam dimension, and (2) amplitude modulation toenable slope control on ablated features. Slope control is an aspect ofattaining acceptable depth variation at feature intersections.

The AOD frequency and amplitude modulation commands are updated, in thisexample, through an FPGA. For example, FIG. 22 is a block diagramrepresenting AOD control data flow implemented in an FPGA 2200 accordingto one embodiment. The nominal update period is about 260 ns. A basicdither table 2210 may be loaded into the FPGA 2200 on a per-applicationbasis. This dither pattern is scaled and split between the two AOD axesdynamically during processing. In addition, an amplitude modulationcommand controls laser power dynamically.

An artisan will recognize from the disclosure herein that thisarchitecture may be extended to include, for example, sets of differentdither waveforms, galvo error correction, and/or profiling assistance.

(b) General Definitions

The XY beam positioning axes are not aligned with the AOD axes (due tothe angle of the turn mirror inside the galvo block). Thus the AOD axesin this example are referenced as axis 0 and 1, rather than X and Y.

Dither: The process of rapidly changing the frequency command to one orboth AOD axes. The location of the process beam on or in the workpieceis a linear function of the AOD frequency command.

F0: AOD frequency command, axis 0.

F1: AOD frequency command, axis 1.

Fnom: Nominal AOD frequency command for zero deflection (nominally 110MHz).

Fd[1 . . . Nd]: A set of deflection frequencies comprising the “dithertable” 2210 described above.

Nd: Number of deflection frequency points.

Kw: Dither width scaling factor. Kw=0 for no dither (nominal processbeam).

W0: Nominal width (for an undithered process beam).

Wmax: Maximum width for full AOD deflection.

Lwc: Length of a width-change transition.

Wk: Width at the end of process segment k.

Theta0: Dither angle offset. Routs cut at this angle relative to thesystem XY axes deflect AOD1 to widen the trench. Angles are definedrelative to the X axis, positive CCW.

Vx=X axis velocity.

Vy=Y axis velocity.

Dosage: The basic process parameter that determines trench quality;defined as power/velocity, or W/(m/sec)=(J/sec)/(m/sec)=J/m. Duringprocessing, dosage may be defined for the nominal (undithered)processing beam at focus.

(c) AOD Frequency Updates

The following equations describe the process of generating the AODfrequency commands during processing according to an example embodiment.Indices are used for variables that are streamed from the SCC or pulledfrom data tables, as noted below. Non-indexed variables are calculatedor measured. In this example, there are “j” process segments (variabletiming) and “k” updates from the FPGA 2200 (nominally 260 ns).

Note that the DSP and FPGA 2200 update periods are nominal and may beadjusted (e.g., in the range of 0.1 to 5.0 μsec), depending onthroughput (for the DSP) or AOD driver performance (for the FPGA 2200).

(c-i) Straight Line Processing

For W[j]=width for the current process segment,

Kw=(W[j]−W0)/(Wmax−W0),

Theta=atan(Vy/Vx)−Theta0,

Fo=Fnom+Fd[k]*Kw,

F0=Fo*cos(Theta),

F1=Fo*sin(Theta).

(c-ii) Corner Processing

If the process trajectory changes direction, Vy and Vx changedynamically during the turn segment. The turn is made at (nominally)constant tangential velocity (Vx{circumflex over ( )}2+Vy{circumflexover ( )}2=const). Theta may be interpolated as a function of time,knowing initial and final angles.

The tool path may optionally be constrained to use only smoothedsegments, such that corners are continuous and sine-profiled (or anappropriate approximation to sine profiling), especially for wide lineswhere Kw>0.

(c-iii) Width Change

During a width change, the dither factor Kw is updated in real time.

Let

Tseg=the accumulated time spent processing a width-change segment (afunction of time),

V=V[j]=V[j−1]=process segment velocity,

Lwc=length of width-change process segment.

The real-time width as a function of time may be

W=W[j−1]+(W[j]−W[j−1])*Tseg/(Lwc/V[j]),

and the dither factor Kw may then be

Kw=(W−W0)/(Wmax−W0).

(d) Power Modulation

(d-i) Operational Modes

Power modulation in the AOD subsystem accomplishes several objectives,including: beam “shuttering” (on/off); power modulation duringprocessing at moderate update rates (about 0.1 to 5 μs); and powermodulation during deflection dithering at high update rates (about 260to 2000 ns).

Beam “shuttering” in this example is provided through the driver'sdigital modulation discrete input (DMOD). Power modulation for processcontrol and dithering is provided through the AOD driver's analogmodulation input (AMOD).

Shuttering may be used for on/off control to provide substantially fullextinction (the analog modulation may leak power even with zero command)This is useful, for example, for mode-locked lasers without Q-switchcontrol.

Process power modulation is intended to maintain the desired dosage(power/velocity ratio) for workpiece features as velocity changesdynamically, or to shape the endwall slopes of features atintersections.

Dithering power modulation serves at least two purposes: (1) tonormalize diffraction efficiency as a function of deflection command,and (2) to shape the beam intensity profile as a function of deflectionposition, which can be used to create controlled slopes at sidewalls.Slope control (specifically, reduced slope angles) improves depthcontrol at the intersections of ablated features.

(d-ii) Process Power Modulation

Power is modulated as function of dosage and velocity. Velocity variesduring acceleration and deceleration segments, and dosage may vary insegments that transition between the dosages of two segments withdifferent widths, or to transition to a different depth:

Dose=Dose[j−1]+(Dose[j]−Dose[j−1])*Tseg/(Lwc/V[j]).

And process power is modulated by the product of dosage and velocity:

P=Dose*V.

Power may be controlled through attenuation at the AOD. The AOD may becalibrated to linearize its attenuation response. The maximum(unattenuated) power level Pmax may be measured before processing apart. Thus:

Atten=P/Pmax.

(d-iii) High-Speed Power Modulation (“Shaping”)

High-speed amplitude modulation, synchronized with the positioningdither, tailors the intensity in order to profile the sidewall slopecontrol to support accurate intersection processing. This is known as“shaping” the intensity profile of the beam. Note that this modulationshould not be confused with the high-speed linearization compensationdiscussed herein because they may be two separate steps. Due to thedesired speed (e.g., an update rate of about 260 ns), high-speedmodulation is implemented in FPGA look-up tables (LUT).

A scaling factor, Ks, scales the intensity shaping, similar to thedeflection dither described above. The scaling allows shaping to be usedonly for intersections, thus avoiding or reducing significant power lossduring long runs of wide features.

Given

Nd=number of entries in the dither table 2210 (e.g., about 256 entries),

Shape[k]=table 2212 of shaping values (0 to 1; Nd entries),

Ks=shaping scale factor (0 to 1; updated at a 0.1 to 5 μs rate).

The scaling of the shaping table 2212 is then given by:

Kshape=1−Shape[k]*Ks (fork continuously cycling between 1 and Nd).

Note that a Shape[ ] table of all 1's 2213 actually corresponds to zeropower (for Ks=1). Thus, the table may be loaded with (1−NomShape), wherethe user specifies NomShape=1 for full power and 0 for zero power.

(d-iv) AOD Response and Linearization

The power modulation modes described above assume that the amplitudemodulation command produces a linear response in beam power. However,the response of the AOD to the amplitude modulation command may benon-linear and, therefore, may be normalized.

The AOD attenuates the output beam power when its RF drive amplitude isvaried (with the RF frequency determining deflection amplitude). Theattenuation characteristic is approximated by a sine function, with thepeak transmission (minimum attenuation) at some value of the amplitudemodulation command. This relation is illustrated by a “diffractionefficiency” (DE) curve, similar to that shown in FIGS. 20 and 21. For asingle RF frequency (thus fixed deflection angle), this modulation curvemay be linearized through a single (LUT).

A complication may arise when different RF frequencies (deflectionpositions) are used. The peak of the DE curve can occur at a differentmodulation command, and the DE peak value may vary as a function of RFfrequency. While this effect may be minor (e.g., on the order of about10% to about 20% for small deflection ranges, on the order of 5-10 spotdiameters), it may be large enough to be of concern for some processingapplications.

To normalize this properly, a unique amplitude modulation correction LUTcan be applied for a specific range of frequencies. For certainembodiments, in which the deflection range is moderate (e.g., aboutthree to five spot diameters), eight LUTs are sufficient. A set of LUTsmay be used for each AOD axis.

The LUT can provide either a direct mapping to the amplitude modulationcommand, or a linearization scaling factor. The scale factorimplementation can reduce the number of data points and/or the need tointerpolate. A scaling implementation is shown in FIG. 22.

Because certain AOD operational modes may use fast (e.g., about 4 MHz)amplitude modulation, the LUT correction may be applied (at the FPGAlevel) for each frequency command update.

(d-v) Amplitude Modulation Summary

In summary, amplitude modulation may be generated by the product ofthree attenuation factors: (1) process power, (2) fast amplitudemodulation (synchronized with position dithering), and (3)frequency-dependent linearization. The complete power modulationsequence can be described as follows: a process power modulation value(Kpwr) is specified, based on dosage and velocity(power=dosage*velocity) once per process update period; a frequencycommand is calculated from the dither table 2210 (once per dither updatecycle); a high-speed amplitude modulation value (Kshape) is calculated(once per dither update cycle); the desired attenuation command is givenby AttenDes=Kpwr*Kshape; and an amplitude modulation command isgenerated by the linearization LUT, based on the current frequencycommand and AttenDes.

FIG. 22 illustrates the FPGA AOD control blocks, including the detailsof position dither, shaping attenuation, low-speed attenuation, andlinearization.

Note that, in certain embodiments, only one AOD may be needed to controlpower. There may be an advantage in keeping the analog modulationcommand constant for one AOD channel (Ch0) because an AOD operates morepredictably in “saturation” or full-scale amplitude modulation. Thesecond AOD (Ch1) controls beam attenuation.

The linearization table 2214 is still used to linearize the opticalefficiency of Ch0. The product of the Ch0 attenuation command (Atten0),the global low-speed attenuation command (Kpwr), and the ditherattenuation (Kshape) is the Ch1 selected attenuation (Anent). This isprocessed by the Ch1 linearization table 2215, creating the Ch1 analogmodulation command

The Ch0 analog modulation command output for Ch0 is held at 1. Thelinearization table 2214 for Ch0 is then a function of a singleparameter (frequency command) The Ch1 linearization table 2215 remains afunction of both selected attenuation and frequency command.

(e) Resolution and Timing

The minimum resolution and update rates for the data described above aresummarized in the table below. Higher resolution or faster update ratesare acceptable. Although not discussed below, if using a laser with apulse repetition frequency near the dither update frequency, the RFfrequency updates may be synchronized with the laser pulse timing.

Latency correction 2216 may be provided to synchronize power control andbeam positioning. Coordination tolerances during intersection processingare on the order of about 1 μm to about 2 μm at the workpiece. Forprocessing velocities of about 1 m/s to about 2 m/s, this uses powercoordination with a resolution of about 1 μs.

There are at least two areas where this coordination is used. In bothcases, the quality of the intersection processing depends on control ofthe wall slopes of the intersecting trenches.

(e-i) Shaping Amplitude Modulation

A purpose of shaping amplitude modulation is to shape the sidewalls of atrench feature with amplitude modulation synchronized with the ditherfrequency. Latency sources such as DAC conversion and/or AOD driverresponse may be accounted for. In one embodiment, the shaping table 2212is “skewed” relative to the frequency dither table 2210 because theupdate period for each is less than one microsecond.

(e-ii) Low Speed Amplitude Modulation

A purpose of the amplitude modulation is general process control, e.g.,dosage control to maintain the desired feature depth. One case is thepower control at the end of an intersecting trench. In this case, thepower is quickly attenuated to create a proper endwall slope at theproper position. For endwall slope control to contribute less than about5% depth variation at intersections, and assuming a 20 μm endwalltransition zone, 1 μm positioning accuracy (due to both timing and beampositioner positioning) is used. If processing at about 1 m/s, this usestiming accuracy of less than about 1 μs.

One approach to intersection processing is to run the beam positionerpast the intersection at constant velocity (to minimize disturbances tothe positioning system), and time the dosage ramp-down such that ittransitions at the correct position. Latency effects may be handled byshifting the transition position by (latency*velocity). But this may berisky because any variations in velocity (e.g., increasing up theprocess velocity) may have a side effect of shifting the transitionpoint. Thus, latency adjustment 2216 is made by adjusting the timing ofthe dosage control relative to the beam positioner.

Both advance and retard adjustment can be performed by delaying the beampositioner position command in multiples of the command update period(Tupdate), and adjusting a dosage delay parameter for fractional delaytimes. The fractional delay can be implemented by an FIR filter on thedosage command before it is transmitted to the AOD subsystem. An exampleof a filter, for a delay less than the update period, is

Dout[k](1−Cd)*Dcmd[k]+Cd*Dcmd[k−1],

where

Dout=latency-corrected dosage command to the AOD subsystem,

Dcmd=dosage command from the command stream,

k=time index,

Cd=delay coefficient=delay/Tupdate,

The following table summarizes the example parameters and data tablesused in the AOD update calculations according to one embodiment.

Minimum Update Resolution Array Variable period Data Source (bits) SizeComment Kpwr  5 μs Embedded 12 Scalar Process power SW attenuationcommand Ks  5 μs Embedded 12 Scalar Shaping amplitude SW scaling (trenchshape) Kw  5 μs Embedded 8 Scalar Dither position scaling SW (trenchwidth) Fd 260 ns FPGA- 8 256 × 1 Dither frequency table command Shape260 ns FPGA- 12 256 × 1 High Speed Amplitude table Modulation (ditheramplitude control) Klin 260 ns FPGA- 12 256 × 8 Amplitude modulationtable linearization correction Atten N/A Loaded 8 Scalar Synchsamplitude Delay once modulation with galvo position Freq N/A Loaded 8Scalar Synchs position dithering Delay once with galvo positioning Fnom260 ns Embedded 8 Scalar Center frequency SW

XIV. Example Intersection Processing Embodiments

(a) Introduction

This section outlines the approach for creating intersecting featuresduring laser processing. This type of processing helps create afunctional laser processing system, and may be a challenge in the systemdesign.

A challenge of intersection processing for the ablated features,according to certain embodiments, is due to a desired control of depthvariation of about ±10%. Reasons for the depth control tolerance includebreakdown voltage (from trace to underlying ground planes), leakage, andimpedance variations.

(b) Basic Approach

The depth of an ablated feature is roughly linear with the accumulatedlaser fluence. Thus double exposure of an intersection may result inabout 100% depth variation. To avoid this, intersections are created byterminating the cutting of a “butting” trench at the point ofintersection. The depth variation using this approach depends on theshape of the processing beam, which can be modified through position andamplitude modulation.

(b-i) Gaussian Beams

The system may cut an intersecting feature such that it ends at anoptimum overlap point that minimizes depth variation. However, theaccumulated fluence of Gaussian beams may make this difficult. FIG. 23illustrates a nominal trench and a butting trench before theintersection is processed. FIG. 24 illustrates the cross-section alongthe butting trench axis shown in FIG. 23. Note that the end slope of thebutting trench is smaller than that of the nominal trench incross-section. This may be due to the accumulated fluence at the head ofthe butting trench. With a slope mismatch, a perfect intersection maynot be able to be created. FIGS. 25 and 26 illustrate the result of theoptimum intersection, which has a depth variation of about ±10%. Theintersection allows no tolerance for position variation.

(b-ii) Dithered Beam

When the process beam is not dithered, the resulting trench slopes aredetermined by the (constant) Gaussian spot diameter. By dithering theprocess beam normal to the trench axis, the side slope can be modifiedsuch that the butting and nominal slopes are nearly equal, and lesssteep (see FIGS. 27 and 28). At the optimum intersection point, thedepth variation is less than about 2% (see FIGS. 29 and 30). However,sensitivity to positioning error may still be unacceptable, e.g., a 1 μmposition error may produce more than 10% depth variation.

(b-iii) Slope Shaping

The dithered beams improve the intersection because the ditheringreduces the slopes of the intersecting process beams. However, theseslopes may be only moderately reduced by dithering the beam positionwhile holding beam power constant. Additional control of the trenchslopes can be provided by modulating the power of the dithered beam as afunction of dither position for the “nominal” trench (to control itssidewall slope), and/or by modulating the power of the beam as thebutting trench approaches the intersection (to control the endwallslope).

Assuming an intersection positioning error tolerance of Ebp, and anallowable relative depth variation of Δ, the width of the slopetransition zone is given by Ws=Ebp/Δ. For example, with Ebp=1 μm andΔ=0.05 (5%), Ws=20 μm.

A simple way to create a controlled slope is to profile the AODattenuation, with a linear ramp from 0 to 1 over the transition zone(using dither modulation for the nominal trench, and position-dependentmodulation on the butting trench). The resulting accumulated fluence isa convolution of the amplitude modulation and the beam profile. A “flattop” portion of the attenuation curve equal to the beam diameter keepsthe transition slope from being influenced by the opposite slope.

FIG. 31 illustrates a nominal and butting trench pair, with a 20 μmwidth on the transition slope. FIG. 32 illustrates the cross-section ofthe trenches before intersection. In this case, only one wall of thenominal trench is sloped, while the profile of the butting trench is notshaped. The shaping may be desirable to minimize throughput impacts.Shaping may be implemented through attenuation and thus reduceseffective laser power and throughput. In cases where the nominal andbutting trenches are formed with different dither patterns, theeffective dosage (J/m) applied to the two trenches is adjusted tonormalize the trench depth.

FIG. 33 illustrates the completed intersection with optimum positioning,while FIG. 34 illustrates the resulting depth variation at the optimumposition and with ±1 μm tolerance. As expected, the 20 μm widthtransition creates about ±5% depth variation given about ±1 μmpositioning error.

(b-iv) Crossed Intersection

The previous example provided good control over the intersection depth,at the expense of a wider trench dimension. This can become a problem ifthe travel range of the dither mechanism is constrained, which may bethe case with AODs. An alternative approach which reduces the dithertravel range is to process the intersection with crossed, rather thanbutted, trenches. Each trench is processed with a symmetrical “notch” atthe intersection. With the notch depth equal to half the nominal trenchdepth, the side slopes can be half as steep for the same trench width.Thus, for the same dither width, the crossed approach is half assensitive to positioning errors as the butting approach.

While crossed intersections generate a short stub on one side of a “T”intersection, according to certain embodiments, the stub's short length(of a few microns) may cause minimal impedance effects. Conversely, thisapproach allows for using identical processing for the two intersectingtrenches and simplifies the processing of crossed (non-“T”)intersections.

FIGS. 35, 36, 37, and 38 illustrate the properties of crossedintersections using notches according to certain embodiments. The sameposition tolerance is achieved while using a dither width half that ofthe previous example.

FIG. 39 illustrates one embodiment of a T intersection, which creates asmall stub (a few microns long) at the intersection. As discussed above,the embodiment shown in FIG. 39 may be created using notches at theintersection. The resulting stub may cause little or no changes toimpedance when the resulting trenches are plated for electricalconduction.

(c) Processing Parameter Definition

In order to properly process intersections, according to certainembodiments, the parameters for position dithering, amplitude dithering,and power control are defined. These parameters depend on variousproperties of the application, such as nominal Gaussian spot size,trench depth, trench width, and process velocity. In the examplearchitecture described above, the high-speed amplitude and positiondither parameters are created using FPGA look-up tables. Creating allintersections with the same dither parameters, according to certainembodiments, avoids excessive complexity with multiple dither tables.This implies that the same dither parameters used for intersectionsapply for generating wide trenches, which may be acceptable. Anotherembodiment includes loading multiple dither tables for different featuretypes.

To process an intersection, at least some of the following steps may befollowed: change velocity to that desired for the intersection (e.g.,based on precision, command update rates, and other processing factors);change width (dither position) and dosage (to maintain depth); changecross-sectional shape (dither amplitude) and dosage (to maintain depth,if required); decrease dosage (create intersection slope); maintaindosage at half of nominal for some distance (this may be optional);increase dosage (create opposite intersection slope); changecross-sectional shape (dither amplitude) and dosage (to maintain depth,if required); change width (dither position) and dosage (to maintaindepth); and change velocity back to nominal.

In certain embodiments, the steps may be completed sequentially andseparately in order to avoid interactions that may affect depth control(e.g., nonlinear power control may be used when combining width andvelocity change).

The diagram of FIG. 40 illustrates the dynamics of dosage and shapecontrol at intersections according to one embodiment.

It will be understood by those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

What is claimed is:
 1. A method comprising: generating a laser beam; passing the laser beam through a first acousto-optic deflector (AOD) and a second AOD; chirping a first RF signal applied to the first AOD; and chirping a second RF signal applied to the second AOD, wherein chirping both the first RF signal and the second RF signal selectively changes the spot size of the laser beam at a surface of a workpiece on a pulse-by-pulse basis to selectively change a width of a kerf cut in the surface of the workpiece by the laser beam
 2. The method of the claim 1, wherein generating the laser beam comprises generating a series of laser pulses, the method further comprising selectively changing the spot size between successively generated laser pulses.
 3. The method of the claim 1, further comprising, after passing the second diffracted laser beam through the scan lens, directing the second diffracted laser beam onto a workpiece to cut a kerf in the workpiece.
 4. The method of the claim 1, wherein the laser beam comprises a pluralityh of pulses, and the generating of at least one selected from the group consisting of the first diffracted laser beam and the second diffracted laser beam is performed on a pulse-by-pulse basis. 